One of the main stories in A Slight Trick of the Mind springs from the elderly Sherlock Holmes's correspondence with a Mr Umezaki concerning prickly ash ("or, as it was called in Japanese, hire sansho") and his expedition to Shimonoseki to seek a specimen and try the cuisine for its supposed longevity-enhancing properties. The seeds are used as a condiment known as Sichuan pepper, which has an unusual pungent yet numbing effect (Holmes has written a monograph on royal jelly that features "Further Comment Upon the Health Benefits of Prickly Ash" - a nice authorial allusion to Holmes's well-known monograph on types of cigar ash).
Anyhow, I see that the linguistics weblog Language Log has a current post by Professor Victor Mair (LL's resident expert on Chinese language and literature) with extensive discussion of prickly ash terminology and cuisine, spinning off from explanation of a rather strange menu item. See Wonton in Zanthoxylum schinifolium etzucc sauce.
Update after reading around the topic a bit:
The whole area of flavours with neurotransmitters is altogether interesting. It's now well-known, at least among foodies, that capsaicin - responsible for the burning sensation of chili peppers - is a long-chain molecule that engages with nerve receptors in the mouth and throat normally associated with heat and inflammation. (The hotness of black pepper is down to a different compound, piperine, that works on the same receptors).
Capsaicin also a low-grade neurotoxin, though the jury is out on how neurotoxic it is, and how this depends on mode of application. Is there any truth, for instance, in the anecdotes of elderly colonials having their taste buds burnt away by decades of curry? Anyhow, it's an ongoing research topic to see if capsaicin can be delivered in a way useful as a radical analgesic that would work by destroying the pain-carrying nerve pathway.
It's also known among foodies that the hotness of wasabi and horseradish has a different mechanism: the release of a more volatile irritant organosulphur compound allyl isothiocyanate, that engages with "TRPA1 and TRPV1 ion channels" (Wikipedia says) in the mucous membranes of the nose and nasopharynx (a tip there for eating wasabi peanuts - don't let the vapour rise).
With the prickly ash family, the story's different yet again. The seeds produce a "tingling paraesthia" (as described by Professor Mair and correspondent Jeff W in the LL thread as tingling, and ultimately major numbness) via a compound called hydroxy alpha sanshool. The rather odd chemical name derives from sanshō (the Japanese species of prickly ash Zanthoxylum piperitum mentioned in the Cullin novel) and the alcohol suffix "-ol". According to the Wikipedia citations, its effects are down to this ...
... newer evidence suggests that the two-pore domain potassium channels KCNK3, KCNK9, and KCNK18 are primarily responsible for hydroxy-alpha sanshool's effects on somatosensory neurons. [ref - Nat Neurosci. 2008 Jul; 11(7): 772–779. doi: 10.1038/nn.2143]... which I guess comes down to it making your tongue think it's being touched, until the receptors get fed up and go away to sulk for a while.
excites D-hair Afferent nerve fibers, a distinct subset of the sensitive light touch receptors in the skin, and targets novel populations of Aβ and C-fiber nerve fibers. [ref - J Neurosci. (Society for Neuroscience) 30 (12): 4353–4361. doi:10.1523 / JNEUROSCI. 4666-09.2010]
- Wikipedia, Hydroxy alpha sanshool, retrieved 9th May 2015
There's a deal more about this in the book Chemistry and Technology of Flavours and Fragrances (David Rowe, John Wiley & Sons, 2009), which goes into the many chemical families that give "tingle activity" and the culinary/medicinal plants where this is a notable effect. They all have a similar structure - which you'd expect for some kind of shape-based interaction with nerve receptors (see A fan letter to the Epicureans). Apart from alpha-sanshool, others include isoaffinin from Ctenium ("toothache grass"); spilanthol from the flowering herb Acmella oleracea (aka toothache plant and paracress, and jambu in Brazil); and pellitorine (commercially isolated from the root of Piper nigrum - black pepper - but also found in Artemisia dracunculoides - Russian tarragon). Food Flavors: Chemical, Sensory and Technological Properties (Henryk Jelen, CRC Press, 25 Oct 2011) has a similar line-up (see pages 26-27).
That tarragon example explains a deal, as it also applies to the closely-related culinary French tarragon Artemisia dranunculus. I remember years back getting an odd metallic-numb sensation on my tongue from chewing a sprig while cutting some up for a chicken - bacon - balsamic - tarragon recipe, and needlessly panicking about whether raw tarragon might be toxic. I now find that this is a well-documented effect - see Google Books for ["tarragon" "numb" "tongue"] - and it's nice to see an explanation for the mechanism, which cookery and herbal books don't go into.
Ah. You don't spot these things for years, and suddenly they're in plain sight when you become aware of their existence. I see the Blue Dragon brand of Kung Po Stir Fry Sauce is billed as "This sauce uses chillies and ground Szechuan peppercorns to create an intensely spicy dish full of flavour". I just got a sachet from the Co-op next door. Whether the Szechuan pepper sensation will be discernible against the general backdrop of chili is another matter - but we'll give it a go. (Edit: we did. The sauce was nice enough - though these things never have enough umami - but I can't honestly say I could discern any difference in sensation from a light dash of chili. I think more experiment is called for).